1. Field of the Invention
The present invention relates to a prism of a finder which is used in a camera, in particular a finder prism which is provided with a shoulder portion so that lines are formed so as to be visible within the field of view of the finder.
2. Description of the Related Art
Conventionally, a finder of a camera is formed with some in-finder information formed on itself, such as automatic focus target marks and short range correction marks. Such marks are formed by virtue of special lines formed in predetermined positions on a surface of a certain plate-like element.
A commonly used means for forming such in-finder information has been disclosed in Japanese Patent No. 2629690, which teaches that desired lines may be directly formed on a glass plate (i.e., a reticle) by means of an etching treatment.
Under the above-mentioned condition, however, since lines are formed on a glass plate by means of an etching treatment, when light passes therethrough, some problems occur which are associated with the depth of the etching treatment. Namely, when light passes therethrough, the reflectance from the surface of the glass plate will be different from the reflectance from the etched portions. As a result, it will appear to a person using the finder as if each single line is instead two lines.
In order to solve the above problem, Japanese Patent Publication 5-22890 has disclosed an improved finder prism in which the cross sections of convex portions (i.e., protrusions) and the cross sections of concave portions (i.e., grooves) are semi-circular, and the surfaces of the convex and concave portions are made to include a light dispersing structure.
However, when the cross sections of the convex portions and the concave portions are made into generally semi-circular shapes the corresponding manufacturing process becomes too complex, and thus the production cost is high.
Further, since the cross sections of the convex portions and the concave portions are substantially circular and since it is these cross sections that are then etched so as to be made visible to the user, it becomes difficult to avoid a problem of the width of the lines being large.